Amplifier circuits often have the problem that they contain temperature-dependent components, the consequence of which is that the output signals of the amplifier circuit are likewise a function of the operating temperature.
FIG. 1 shows a schematic circuit diagram of an amplifier circuit based on CMOS (Complementary Metal Oxide Semiconductor) technology. The amplifier circuit contains a transconductance amplifier OTA, which converts an input voltage VIN into a current I2. The current I2 is converted into an output voltage VOUT by an operational amplifier OPA, which is connected up with a resistor R1 as a current-voltage converter CVC.
In addition to high-impedance inputs 1 and 2, the transconductance amplifier OTA also has a high-impedance output 3. The transconductance amplifier OTA therefore behaves like a current source so that the current I2 that can be coupled out at the output 3 is controlled by the input voltage VIN in accordance with the following equation (1):I2=S*VIN,  (1)where S denotes the transconductance of the transconductance amplifier OTA and is given by the differential of the current I2 with respect to the input voltage VIN at the operating point.
Within the transconductance amplifier OTA, the latter has a differential amplifier stage comprising a current source IREF and also transistors T1 and T2. The input voltage VIN is present at the gate terminals of the transistors T1 and T2. Furthermore, the transconductance amplifier OTA comprises three current mirrors constructed with transistors T3 and T4, T5 and T6, and T7 and T8. The transistors T1, T2, T7 and T8 are n-channel MOSFETs, while the transistors T3, T4, T5 and T6 are p-channel MOSFETs.
The current I2 is converted into the output voltage VOUT by the operational amplifier OPA by means of the resistor R1 in accordance with the following equation (2):VOUT=R1*I2+VCM,  (2)where VCM specifies the center voltage. A combination of equations (1) and (2) yields equation (3) as the transfer function of the present amplifier circuit:VOUT=S*R1*VIN+VCM,  (3)where the product of the transconductance S and the resistance R1 specifies the gain factor of the amplifier circuit.
In CMOS fabrication processes, linear resistors are often produced by deposition of polysilicon material. The temperature coefficient of such resistors which specifies the change in resistance with temperature, is correlated with the resistance per unit area of the polysilicon. In the case of only small resistances per unit area, the temperature coefficient is positive. The temperature coefficient decreases as the resistance per unit area rises and becomes negative for large resistances per unit area.
In the case of the present amplifier circuit, the resistor R1 must have a high resistance. Otherwise, the transconductance S of the transconductance amplifier OTA would have to be large in order nonetheless to obtain an acceptable gain factor of the amplifier circuit. However, this would in turn entail an unacceptably large current consumption of the transconductance amplifier OTA.
A high-value resistor R1 is fabricated by means of polysilicon with a high resistance per unit area in CMOS technology in order to avoid an excessively large area consumption. What is disadvantageous about this, however, is the resulting negative temperature coefficient of the resistor R1. Since the transconductance S of the transconductance amplifier OTA likewise decreases as the temperature rises the gain factor of the present amplifier circuit is greatly temperature-dependent. This property of the amplifier circuit is particularly disadvantageous if the amplifier circuit is operated in a wide temperature range. Moreover, fabrication tolerances of the high-value resistor R1 also influence the gain factor.
The present amplifier circuit is also utilized to amplify AC voltages. The open-circuit frequency of the amplifier circuit likewise depends on the gain factor. In the case of a gain factor that is unstable over a certain temperature range, this leads to stability problems in the amplifier circuit.
Previous solutions to the abovementioned problems comprise the use of a resistor R1 with only a small temperature coefficient and low fabrication tolerances and also the use of a temperature-independent current source IREF, thus resulting in only a low temperature-dependent transconductance S of the transconductance amplifier OTA. Although the temperature dependence of the gain factor is minimized by this solution approach, the area of the circuit becomes large because it is necessary to use polysilicon resistors with a low resistivity, or else it is necessary to produce temperature-stable high-value resistors by means of additional process steps. Both measures are complicated and cost-intensive.
In a further solution approach, in contrast to the solution approach just described, the temperature independence of the current source IREF is dispensed with and only a resistor R1 with a small temperature coefficient and low fabrication tolerances is used. However, the amplifier circuit is permitted to be operated only in a relatively small temperature range in this case.